Short-circuit apparent power of the system

When doing power systems study, it always the predicament of Electrical Engineers where to get the values of the utility short circuit MVA. I was just talking to a senior engineer a few weeks back and he told me that he always consider an infinite bus supply when doing his studies. Despite the fact that it is partially correct, if the fault current values are used in the selection of equipment, then it will be very expensive.

The available short circuit varies significantly from location to location even for a particular utility, thus the utility provides the more accurate values for these electrical parameters. It is not always an easy tasks for the electrical engineer though to get the utility line parameters. Talking to the utility can sometime be very difficult and stressful. If you ask me, I would rather not do that unless I know anybody with in their organization.

If you faced a blank wall, as an electrical engineer tasks to size electrical equipment, what will you do? Use infinite bus as an assumption? No way, especially if you are trying to reduce the cost of your project.

It is here where IEC 60076-5 Power transformers – Part 5: Ability to withstand short circuit comes into rescue. Table 5 of this standard provides the short-circuit apparent power of the system for any particular voltage. As a bonus, it provides for both current European and North American practice.

Table 2 - Short-circuit apparent power of the system

Highest voltage for equipment, UmkV

Short-circuit apparent power, MVA

Current European practice

Current North American practice

7.2, 12, 17.5 and 24

500

500

36

1 000

1 500

52 and 72.5

3 000

5 000

100 and 123

6 000

15 000

145 and 170

10 000

15 000

245

20 000

25 000

300

30 000

30 000

362

35 000

35 000

420

40 000

40 000

525

60 000

60 000

765

83 500

83 500

Source: IEC 60076-5
For Australia, use AS 2374.5-1982 Power transformers - Ability to withstand short-circuit which is a modified version of the IEC standard to suit Australian practice.

In above table, it is very noticeable that North American practice have higher fault MVA ratings for their equipment thus significantly larger in size.